1. Field of the Invention
The present invention concerns molding optical lenses, in particular ophthalmic lenses, from a polymerizable synthetic composition.
2. Description of the Prior art
The traditional mold comprises two pieces, typically made of mineral glasses, at the periphery of which is disposed an annular closure member, such as a gasket or an adhesive tape, defining with them the required molding cavity.
The conventional way to fill a two-piece mold as above is by causing the molding material to flow into the molding cavity through a casting opening provided for this purpose in the closure member.
In at least partly automated process, the mold to be filled is vertically aligned with a filling device adapted to deliver a particular quantity of molding material through a nozzle.
One possibility with such a vertically oriented mold is to fill the molding cavity by gravity only through a nozzle disposed on the top of the mold periphery. See for example U.S. Pat. No. 5,110,514.
Experience has shown that unless special measures are implemented, for example a relatively long waiting time (in the order of two hours, for example) between filling the mold and starting polymerization, the optical lenses obtained frequently have localized optical defects of greater or lesser severity, leading to relatively high reject rates.
To overcome the above problems, U.S. Pat. Nos. 5,547,618 and 5,662,839 propose filling the molding cavity of a two-piece mold vertically oriented through a casting opening formed in a lower part of the closure member and using pumping means adapted to cause the molding material to flow without significantly pressurizing it.
Typically, filling of the molding cavity is effected using a pipe connected to a pressurized molding material reservoir and applied, like a filling nozzle, to the casting opening.
The annular closure member, for example an adhesive tape, must not only fixedly maintain the two pieces of the mold in the required relative positions but also provide a leak-proof seal with regard to the liquid polymerizable material for avoiding leakage of the material of the periphery of the mold.
Conventional adhesive tapes used to form the annular closure members are generally impervious both to the liquid polymerizable material and gases.
During the filling of the molding cavity with the liquid polymerizable material, over filling of the molding cavity shall be avoided in order not to deteriorate adhesion of the peripheral adhesive tape while gases shall be evacuated from the mold cavity and entrapping of gas bubbles within the liquid polymerizable material shall be avoided as much as possible.
Thus, when using a conventional peripheral adhesive tape as the annular closure, an opening must be provided in an upper position of the tape to allow venting of the gases. This venting opening is typically formed by punching the adhesive tape with a needle.
One of the main problems is to evacuate the remaining gas from the mold without leakages of excess monomer by the venting opening. In particular, when using a bottom filled mold, it is difficult to vent out the last air bubble which is usually entrapped at the top of the molding cavity, just beneath the punctured portion of the adhesive tape. This is particularly the case when molding negative power glasses.